Wafers find a variety of uses in the semiconductor, solar energy and other industries. Wafer quality often depends on variables such as thickness and surface characteristics. Depending on end use, poor quality wafers may have uneven thickness or uneven surface characteristics; whereas, higher quality wafers may have substantially uniform thickness and substantially uniform surface characteristics. In the semiconductor industry, where wafers are used as a substrate, wafer quality can crucially influence mechanical and/or electronic yield of wafer-based semiconductor circuits.
Wafer thickness can affect mechanical, electronic and optical behavior or performance. With respect to mechanical performance, increased thickness can minimize detrimental effects of fabrication or use associated stresses. With respect to electronic and optical performance, wafer thickness is often an important variable. Further, wafer thickness is often defined by distance between an upper wafer surface and a lower wafer surface. Characteristics of one or both of these surfaces generally affect electronic and optical performance.
Industries that rely on wafer-based technologies typically seek to control wafer thickness through any of a variety of conventional manufacturing and measurement techniques. In the semiconductor industry, wafer thickness may be controlled through various processing techniques that involve wafering (sawing), etching, and polishing, which typically produce flat wafers with substantially parallel surfaces. Thickness measurement of such wafers is fairly well defined and straightforward. For example, conventional measurement techniques include use of a dial gauge (e.g., a manual, mechanical measurement, generally used in the laboratory for a small number of wafers) and use of impedance (e.g., measurement of capacitance or eddy current loss in a radio frequency bridge configuration). Some conventional techniques, such as those that rely on impedance, can be implemented in a non-contact manner. For example, a coil may create an electromagnetic field proximate to a wafer, without actually contacting the wafer, and thereby generate one or more eddy currents in the wafer. Information related to the generated current or currents is then used to determine wafer thickness. Various impedance techniques can be quite rapid, on the order of a few seconds for each wafer, but they require knowledge of wafer properties such as resistivity. Further, impedance techniques are not well suited for determining or mapping variations in wafer thickness.
In general, the solar energy industry imposes certain demands on wafer quality. Such demands often need to be amenable to high throughput. For example, a typical manufacturing facility may process 50,000 wafers per day wherein a reasonable fraction of these wafers must be subject to measurement techniques to provide meaningful quality assurance. Concomitantly, such measurement techniques should be quite rapid. Other demands relate to wafer morphology, noting that wafers used in the solar energy industry are typically not flat. For example, silicon-based ribbons can exhibit surfaces that are substantially non-parallel with substantial variations in surface morphology. In such instances, an “average” thickness measurement and/or a thickness profile is useful. Yet other demands concerns crystallinity, i.e., the crystalline or multi- (or poly-) crystalline nature of wafers. Conventional impedance techniques can produce inaccuracies for multi- or poly-crystalline wafers because of extraneous impedance associated with grain boundaries and defects. Surface characteristics impose other demands because wafers used in the solar energy industry typically have some substantial degree of surface texture that acts to reduce surface reflectance and maximize optical absorption. Various aspects of surface texture can lead to inaccuracies in determination of thickness and/or surface characteristics. For example, surface roughness can be particularly detrimental for determinations based on optical measurement techniques.
Surface texture includes roughness and waviness, in addition, many surfaces have lay (e.g., directional striations across the surface). Roughness includes the finest (shortest spatial wavelength or spatial periodicity) irregularities of a surface. Roughness generally results from a particular production process or material condition. Waviness includes more widely spaced (longer spatial wavelength or spatial periodicity) deviations of a surface from its nominal shape. Waviness errors are intermediate in wavelength between roughness and form error. Note that the distinction between waviness and form error is not always made in practice, and it is not always clear how to make it. Lay refers to the predominant direction of the surface texture. Ordinarily lay is determined by the particular production process and geometry used. For example, turning, milling, drilling, grinding, and other cutting tool machining processes usually produce a surface that has lay: striations or peaks and valleys in the direction that the tool was drawn across the surface. Other processes produce surfaces with no characteristic direction: sand casting, peening, grit blasting, etc. Sometimes these surfaces are said to have a non-directional, particulate, or protuberant lay.
Clearly, the above demands are difficult to meet. The difficulties are further intensified by a need for minimizing the equipment and the measurement costs. The solar cell industry typically uses wafer weighing as a technique for monitoring wafer thickness while impedance-based techniques are often used to measure thickness of a very small number of total wafers in a single wafer production facility.
The solar energy industry has also identified a need to measure the spatial variations in wafer thickness. It has been observed that certain processes are affected by the local variations in the wafer thickness. For example, a process in solar cell fabrication involves firing a metal pattern to produce a low-resistivity contact. This process consists of screen printing a contact and spike-firing it at a temperature of about 800° C. in an infrared furnace. In this type of a furnace, the temperature acquired by a wafer depends on wafer thickness. Thus, a wafer that is thicker than the standard wafer (for which the process is optimized) will rise to a higher temperature. This can lead to the metal punching through a junction which will concomitantly degrade the open circuit photo-voltage (e.g., Voc) and fill factor of the solar cell. On the other hand, a thinner wafer may not be sintered properly, resulting (again) in a low fill factor. Of course, similar behavior may occur within different regions of a wafer if wafer thickness is not uniform.
Thus, within the solar energy industry and other industries, a need exists for methods, devices and/or systems for measurement of and determination of wafer variables (e.g., mechanical properties, mechanical behavior, electrical properties, electrical behavior, optical properties, optical behavior, etc.). In particular, a need exists for methods, devices and/or systems that can yield meaningful results for wafers that may have substantial texture (e.g., roughness, etc.) and/or substantially non-parallel surfaces. Further, a need exists for rapid, accurate, and/or low-cost methods, devices and/or systems. Various exemplary methods, devices and methods disclosed herein address aforementioned needs and/or other needs.